HBV afflicts 400 million individuals worldwide and causes an estimated 600,000 deaths each year from complications arising from HBV infection. While several antiviral treatments are approved for use, none of these is able to elicit a therapeutically effective immune response capable of providing durable control of infection except in a small fraction of patients undergoing treatment.
HBV infection results in the production of two different particles: 1) the infectious HBV virus itself (or Dane particle) which includes a viral capsid assembled from the HBV core antigen protein (HBcAg) and is covered by the HBV surface antigen (HBsAg) and 2) subviral particles (or SVPs) which are high density lipoprotein-like particles comprised of lipids, cholesterol, cholesterol esters and the small and medium forms of the HBV surface antigen (HBsAg) which are non-infectious. For each viral particle produced, 1,000-10,000 SVPs are released into the blood. As such SVPs (and the HBsAg protein they carry) represent the overwhelming majority of viral protein in the blood. HBV infected cells also secrete a soluble proteolytic product of the pre-core protein called the HBV e-antigen (HBeAg).
HDV uses HBsAg to form its viral structure (Taylor, 2006, Virology, 344: 71-76) and as such, HDV infection can only occur in subjects with concomitant HBV infection. While the incidence of HDV co-infection in asymptomatic HBV carriers and chronic HBV-related liver disease is low in countries with a low incidence of HBV infection, it is a significant complication in HBV-infected subjects in countries with a high incidence of HBV infection and can increase the rate of progression of liver disease to liver cirrhosis. The unmet medical need in HBV infection is even more pressing in HBV/HDV co-infected subjects; there is no specific approved agent that directly targets the HDV virus and patient response even to combination therapy with approved agents for HBV treatment is poorer than patients in with HBV monoinfection (Wedemeyer et al., 2014, Oral abstract 4, 49th Annual Meeting of the European Association for the Study of the Liver, April 9-14, London, UK).
The current approved treatments for HBV include interferon-α or thymosin α1-based immunotherapies and the suppression of viral production by inhibition of the HBV polymerase by nucleoside/nucleotide analogs. HBV polymerase inhibitors are effective in reducing the production of infectious virions but have little to no effect in reducing HBsAg or only very slowly reduce HBsAg with long term treatment in a limited number of patients (Fung et al., 2011, Am. J. Gasteroenterol., 106: 1766-1773; Reijnders et al., 2011, J. Hepatol., 54: 449-454; Charuworn et al., 2014, Poster abstract 401, 48th Annual Meeting of the European Association for the Study of the Liver, April 24-28, Amsterdam, The Netherlands). The primary effect of HBV polymerase inhibitors is to block the transformation of pre-genomic viral mRNA into partially double stranded DNA, which is present in infectious virions. Interferon based immunotherapy can achieve a reduction of infectious virus and removal of HBsAg from the blood but only in a small percentage of treated subjects.
HBsAg in the blood can sequester anti-HBsAg antibodies and allow infectious viral particles to escape immune detection which is likely one of the reasons why HBV infection remains a chronic condition. In addition HBsAg, HBeAg and HBcAg all have immuno-inhibitory properties as discussed below and the persistence of these viral proteins in the blood of patients following the administration of any of the currently available treatments for HBV as described above likely has a significant impact in preventing patients from achieving immunological control of their HBV infection.
Although the three primary HBV proteins (HBsAg, HBeAg and HBcAg) all have immuno-inhibitory properties (see below), HBsAg comprises the overwhelming majority of HBV protein in the circulation of HBV infected subjects and is likely the primary mediator of inhibition of the host immune response to HBV infection. While the removal of HBeAg, appearance of anti-HBe or reductions in serum viremia are not correlated with the development of sustained control of HBV infection off treatment, the removal of serum HBsAg from the blood (and appearance of free anti-HBsAg antibodies) in HBV infection is a well-recognized excellent prognostic indicator of antiviral response on treatment which will lead to control of HBV infection off treatment (although this only occurs in a small fraction of patients receiving immunotherapy or HBV polymerase inhibitors). Thus, while reduction of all three major HBV proteins (HBsAg, HBeAg and HBcAg) may result in the optimal removal of inhibitory effect, the removal of HBsAg is essential and its removal alone is likely sufficient to remove the bulk of the inhibition of immune function in subjects with HBV infection.
Another critical feature of chronic HBV infection is the establishment of a stable reservoir of HBV genetic information in the nucleus of infected cells called covalently closed circular DNA (cccDNA). cccDNA exists in multiple copies within the nucleus as an extrachromosomal episome which functions as the transcriptional template for the production of mRNA encoding all viral proteins and immature genomes (pre-genomic mRNA) for the production of new virions. After encapsidation in the cytoplasm, the immature pre-genomic mRNA is converted into a mature, partially double stranded DNA genome by the HBV polymerase (which is co-encapsidated with the pregenomic mRNA), thereby rendering the mature HBV genome competent to establish or replenish a cccDNA reservoir in naïve or previously infected cells. The end of the infectious process consists of the delivery of this partially double stranded genomic HBV template into the nucleus and its conversion to cccDNA.
cccDNA can be replenished in the nucleus of infected cells via nuclear import of HBV capsids containing mature HBV genomes which replenish the cccDNA copy number. This nuclear cccDNA replenishment is accomplished by two mechanisms: direct nuclear import of assembled capsids from the cytoplasm or re-infection of previously infected hepatocytes with subsequent shuttling of the internalized capsids into the nucleus (Rabe et al., 2003, Proc. Natl. Acad. Sci. USA, 100: 9849-9854). The transcriptional inhibition or elimination of this genomic HBV reservoir in the nucleus is critical to the establishment of long term control of HBV infection following treatment.
Long term treatment with nucleoside/nucleotide HBV polymerase inhibitors can reduce cccDNA copy number within the nucleus, consistent with the ability of HBV polymerase inhibitors to block replenishment of cccDNA by nuclear import of capsids containing mature HBV genomes. However, while the cccDNA copy number per hepatocyte is reduced, it still remains transcriptionally active thus HBsAg levels remain largely unaffected (Werle-Lapostolle et al., 2004, Gastroenterol., 126: 1750-1758; Wong et al., 2013, Clin. Gastroenterol. Hepatol., 11: 1004-1010; Wong et al., 2014, Poster abstract 1074, 49th Annual Meeting of the European Association for the Study of the Liver, April 9-14, London, UK). cccDNA can be transcriptionally inactivated by immune-mediated processes (Belloni et al., 2012, J. Clin. Inv., 122: 529-537) but the ability of the immune response to provoke cytokine responses required for cccDNA inactivation is likely blocked by persistently circulating HBsAg as described in U.S. 2014/0065102 (which is incorporated herein by reference in its entirety) and is consistent with the ineffectiveness of immunotherapies in treating HBV infection.
As such, there exists a clear unmet medical need for a treatment regimen which can elicit a durable immunological control of HBV infection in a large proportion of patients receiving this treatment.